This invention relates to a fluid displacement apparatus, and more particularly to a scroll type fluid displacement apparatus.
Scroll type fluid displacement apparatus are well known in the prior art. For example, U.S. Pat. No. 801,182 (Creux) discloses a device including two scroll members each having a circular end plate and a spiroidal or involute spiral element. These scroll members are maintained angularly and radially offset so that both spiral elements interfit to make a plurality of line contacts between their spiral curved surfaces to thereby seal off and define at least a pair of fluid pockets. The relative orbital motion of the two scroll members shifts the line contacts along the spiral curved surfaces and, therefore, the fluid pockets change in volume. Since the volume of the fluid pockets increases or decreases dependent on the direction of the orbital motion, the scroll type fluid displacement apparatus is applicable to compress, expand or pump fluids.
Referring to FIGS. 1a-1d, the principle of operation of scroll type fluid apparatus, particularly the compression operation, will be described.
FIGS. 1a-1d may be considered end views of a compressor wherein the end plates are removed and only spiral elements are shown at orbital angular positions spaced 90.degree. from one another. Two spiral elements 1 and 2 are angularly offset and interfitted with one another. As shown in FIG. 1a, the orbiting spiral element 1 and fixed spiral element 2 make four line contacts as shown at four points A-D. A pair of fluid pockets 3a and 3b are defined between line contacts D, C and line contacts A, B as shown by the dotted regions. The fluid pockets are defined not only by the wall of spiral elements 1 and 2, but also by the end plates from which these spiral elements 1 and 2 extend.
Orbiting spiral element 1 is moved in relation to fixed spiral element 2 so that the center 0' of orbiting spiral element 1 revolves around the center 0 of fixed spiral element 2 at a radius of 0-0', while the rotation of orbiting spiral element 1 is prevented. This motion angularly and radially shifts fluid pockets 3a and 3b toward the center of the interfitted spiral elements, to gradually reduce the volume of each fluid pocket 3a and 3b, as shown in FIG. 1a-1d, thereby, compressing the fluid in each pocket.
In typical operation, fluid pockets 3a, 3b are initially formed when the ends of spiral elements 1,2 contact with the outer surface of the other spiral elements, as shown in FIG. 1a. Further rotation of orbiting spiral element 1 causes the pockets 3a, 3b to reduce in volume, as shown in FIGS. 1b, 1c. Thereafter, the pair of fluid pockets 3a, 3b become connected to one another, as shown in FIG. 1d, and the single pocket is further reduced in volume, as shown by the undotted central area in FIGS. 1a, 1b and 1c. During the reduction in volume of pockets 3a, 3b, the ends of the spiral elements leave contact with the outer surface of the other spiral elements, as shown in FIGS. 1b, 1c, 1d, until contact is reestablished, as shown in FIG. 1a to form a new pair of fluid pockets 3a, 3b.
This operation results in compression of the fluid in the pockets, since circular end plates are affixed to the axial facing ends of spiral elements 1 and 2. Discharge of the compressed fluid occurs through a centrally located discharge port in one of the end plates, shown diagrammatically as 4 in FIGS. 1a, 1b and 1c.
In comparison with conventional fluid displacement apparatus of the piston type, a scroll type fluid displacement apparatus has several advantages, such as continuous transfer of the fluid, volume efficiency, and relatively silent operation.
However, in order to increase the compressive capacity and compression ratio, the number of turns, or revolutions of each spiral element must be increased. Consequently the diameter of the apparatus also must be increased. This becomes a problem in a scroll type fluid displacement apparatus which is used as a refrigerant compressor of an automotive air conditioner, because the diameter of compressor housing must be kept as small as possible in order to fit the compressor within the typically very narrow space of an engine compartment. Furthermore, both scroll members must be maintained angularly and radially offset, and the dimensional accuracy of the compressor parts must be maintained, or the total dimensional error of the assembled compressor parts must be minimized in order to assure the stability and efficiency of the apparatus.
A cylindrical housing is an advantageous configuration for containing a pair of scroll members each of which have a wall thickness t and outermost angle .phi. of the center line of the scroll wall. An optimal disposition of the end plate and spiral element to reduce the diameter of the housing is disclosed in U.S. Pat. No. 4,304,535 (Terauchi), the disclosure of which is incorporated herein. Accordingly, as shown in FIGS. 2 and 3 herein, orbiting scroll 23' orbits at radius Ror while maintaining its angular orientation with fixed scroll 22'. The sectional area of the housing needed to permit the orbital motion of orbiting scroll 23' at radius Ror will be determined by the spiral or snail shaped area occupied by fixed spiral element 222' (area D in FIG. 3) and the space (area B in FIG. 3) over which orbiting spiral element 232' is swept. Therefore, the inner diameter of the cylindrical housing in which the pair of scrolls are contained will be given by 2.phi.rg+t+Ror, where rg is involute generating circle radius. In this construction, the center of inner wall of cylindrical housing is radially offset from the center of involute generating circle of fixed spiral element 222' also the maximum diameter of orbiting end plate 231' to permit the orbital motion within the above cylindrical housing will be given by 2.phi.rg+t-Ror.
A suitable drive point of the orbiting scroll is the involute generating circle center of the orbiting spiral element, since the relation between the center of tangential gas force in the fluid pockets defined by both spiral elements and the drive point does not change at any rotational angle of the drive shaft. Accordingly, it is considered to be a normal design criteria to locate, as shown, in FIG. 2 the center of drive shaft Os at the same point as the involute generating circle center O.sub.F of fixed spiral element 222', as a result the drive point O.sub.D of orbiting scroll 231' is automatically disposed on the involute generating circle center Oo of orbiting spiral element 232'. Orbiting scroll 231' therefore revolves at a radius O.sub.F -Oo.
FIG. 4 illustrates the conventional relationship between the center of each end plate and the centers of the involute generating circles of the spiral elements. In this figure, the center O.sub.E of orbiting end plate 231' is radially offset from the involute generating circle center Oo of orbiting spiral element 232' to the right by a distance 1/2 Ror. Also, the center Oc of fixed end plate 221', i.e., the center of the compressor housing is radially offset from the involute generating circle center O.sub.F of fixed spiral element 222' to the right by a distance 1/2 Ror. The center Oc is also radially offset from the center Oo by a distance 1/2 Ror. The drive point O.sub.D is disposed on the center Oo and the center Os of drive shaft 13 is concentric with center O.sub.F, whereby the orbital motion of the orbiting scroll 23' is shown as the locus of the center Oo. The locus of the center Oo is shown in FIG. 4 by the circle C with its center at center O.sub.s of drive shaft 13'.
FIG. 5 is a vertical sectional view of a scroll type compressor which utilizes the above mentioned disposition of scroll members. In this construction, the center O.sub.F of the drive shaft 13' is radially offset from the center Oc of housing 10' and also the center Oo of the drive point is radially offset from the center O.sub.E of orbiting end plate 231'. Therefore, a tubular boss 233' projecting axially from one end surface of end plate 231' and rotatably supported on a drive pin of the driving mechanism is radially offset from the center of end plate 231'. Since tubular boss 233', must be machined at an offset position, the machining process of orbiting scroll 23' is complicated, and hence, it is difficult to form the scroll with high accuracy in relation to the location of drive point and the scroll curve. Furthermore, a coupling mechanism 24' is required to maintain the angular relationship between both scrolls and to carry the tangential gas force from orbiting scroll 23'. However in this construction, the coupling mechanism, such as a ball coupling/thrust bearing device 24', must be disposed within the housing 10' at a radially offset position. The stationary ring member 241' is fitted on the inner end surface of the housing 10' at a radially offset position, thereby a dead space A exists at the outer peripheral portion of ring member 241' when the ring member is formed by two concentric circles. Hence, the diameter of housing 10' should be increased, as shown in FIG. 5.